The present invention is directed to a power supply and data signal interface circuit for a data processing device and, more particularly, to a power supply and data signal interface circuit with overvoltage protection.
A power supply and data signal interface circuit can be connected to an external line for conducting power and data signals between a data processing device and external circuitry. Power and data signal interface circuits are designed and standardized for a specified range of electrical characteristics to enable the interface circuits to operate without risk of damage. However, technical progress and widespread adoption enlarge the field of use to circumstances that were not originally envisaged such that it is necessary to provide protection in the interface. An example is the voltage of the power supply and protection of the device and the interface itself from overvoltage, that is to say an externally forced power supply voltage exceeding the normal specification.
An example of a power and signal interface is the Universal Serial Bus (‘USB’) interface. The USB interface specification was developed to establish communication between peripheral devices and a host controller such as a personal computer. The host controller also provides a power supply for the peripheral device. USB interfaces are now used in a variety of devices such as mice, keyboards, digital cameras, printers, personal media players, flash drives, cell phones, video game consoles and external hard drives. For many of these devices, the USB is the standard connection method.
Typically, a USB connection is made over a cable including power supply lines and data exchange lines. Many portable devices have adopted a USB connector not only for USB data transfer and power supply but also for connecting a battery charger to the portable device. While the originally specified charger or controller is used, there is little risk of overvoltage. However, it is now desired to use a USB connection to connect power supplies or battery chargers to power management integrated circuits (‘PMICs’) of devices where the power supplies or chargers supply a voltage higher than that provided for in the USB specification. More particularly, the USB specification is based on a 5V maximum nominal power supply for a device operating on 3.3V. The Battery Charger addendum to the USB specification specifies battery charging through the USB VBUS pin. Commercial battery chargers may generate power supply at up to 20V. For example, in the automobile industry, an automotive electrical system may have a nominal voltage of 12V and a peak voltage that may reach 20V. Application of such a high voltage to the USB interface constitutes a voltage sufficient to cause severe damage to the interface and to the device in the absence of overvoltage protection.
A number of criteria may complicate the provision of overvoltage protection for a power and data signal interface. For example, the overvoltage protection should not interfere with normal functions in the absence of overvoltage. The overvoltage protection should function effectively even in the presence of fast supply voltage ramp rates. The overvoltage protection also should function even in the absence of an internal power supply for the device, for example if the battery is discharged or if the device is on standby with its internal power supply turned off to save power. Thus, it would be desirable to have a power supply and data signal interface circuit with overvoltage protection that does not interfere with normal operation of the interface circuit.